Tau Boötis

Tau Boötis (τ Boo / τ Boötis) is a yellow-white dwarf approximately 51 light-years away in the constellation of Boötes. The system is also a binary star system, with the secondary star being a red dwarf. As of 1999, an extrasolar planet has been confirmed to be orbiting the primary star.

Planetary system

In 1997 a planet, designated as Tau Boötis b, was discovered orbiting the primary star. There are also some indications of another, more distant planet orbiting the star. In an unusual case of role-reversal, it appears that Tau Boötis' rotation has been tidally locked to Tau Boötis b. The planet was discovered by a team of astronomers lead by Geoff Marcy and R. Paul Butler.

Stellar components

The system is a binary. The primary component is a yellow-white dwarf (spectral type F7 V) and secondary is a dim red dwarf (spectral type M2 V). The system is relatively nearby, distance being about 51 light years. The primary star should be easily visible to the unaided eye under dark skies.

The primary star, Tau Boötis A is a yellow-white dwarf. It is 20% more massive than our Sun and thus is somewhat brighter and hotter. It has a radius 1.9 times solar, and is probably about 1300 million years old. Since it is more massive than the Sun, its lifespan is shorter being less than 6000 million years. Tau Bootis is the first star apart from the sun to be observed changing the polarity of its magnetic field. It is also listed as a suspected variable star. However, the star is not variable.

Tau Boötis B is a dim red dwarf orbiting the primary star at a distance of 240 AU. One orbit around the primary would take thousands of years to complete.


NAME Right ascension Declination Apparent magnitude (V) Spectral type Database references
IDS 13425+1757 B (GJ 527 B) 13h 47m 15.6s +17° 27' 28'' 11.1 M2 Simbad


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